A County Down office has been named the UK's most sustainable building.

The headquarters of construction firm Graham in Hillsborough defeated elite contenders to take the prize. It features a biomass boiler using local fuel and operating at close to zero carbon emissions.

This is the third year in four that a Northern Ireland project has won the sustainability category of the Royal Institution of Chartered Surveyors (RICS) UK awards.

RICS Northern Ireland director Ben Collins said: "It is recognition of the skills of chartered surveyors and other property and construction professionals in Northern Ireland and deserved reward for Graham for what is a fantastic construction project."

The building was designed by architect BDP and was completed at a cost of £7.5 million. Other firms involved in the construction included surveyors Bruce Shaw Partnership, and engineers Caldwell Consulting.

The company said that its headquarters is "built to the highest practicable environmental standards to reflect our underlying commitment in the Group to good design, sustainable construction and creating facilities with a reduced carbon footprint and low operating costs".

It features extensive use of glass to reduce the amount of lighting needed and there is a system for harvesting rainfall.

The building won the sustainability category of the RICS UK Awards at a final event in The Savoy, London, finishing ahead of dozens of prestigious projects from right across the UK.

The refurbishment and extension of the Crescent Arts Centre in Belfast was commended in the community benefit category of the awards.

Graham's success follows in the footsteps of the new Library at Queen's University Belfast, which won the sustainability award in 2010 and The Orchard Building at Stranmillis University College, which took the accolade in 2008.